Impaired glucose homeostasis, as measured by worsened insulin resistance and insulin secretory capacity, accompanies obesity and the metabolic syndrome, ultimately producing hyperglycemia and overt diabetes mellitus. The Framingham Offspring Study recently demonstrated that basal aldosterone concentrations predict the development of the metabolic syndrome, which is associated with increased cardiovascular risk. We have observed that fasting glucose concentrations are decreased in aldosterone synthase-deficient (AS-/-) mice, and that AS-/- mice demonstrate enhanced glucose-stimulated insulin secretion through a potassium-independent mechanism. This proposal tests the central hypothesis that aldosterone attenuates insulin secretion in humans via a mineralocorticoid receptor (MR)-dependent mechanism. The applicant will collaborate with experienced clinical and basic science investigators to gain expertise in the regulation of the rennin-angiotensin-aldosterone system and the field of diabetes, while investigating the following three specific aims: SPECIFIC AIM 1 Test the hypothesis that exogenous aldosterone increases fasting glucose concentrations in humans by attenuating glucose-stimulated insulin secretion. SPECIFIC AIM 2 Test the hypothesis that endogenous aldosterone increases fasting glucose and decreases insulin secretion in individuals with the metabolic syndrome via an MR-dependent mechanism. SPECIFIC AIM 3 Test the hypothesis that MR antagonism and angiotensin II type-1 receptor (AT1) antagonism will cause synergistic beneficial effects on fasting blood glucose and insulin secretion. We will utilize previously-developed protocols for aldosterone infusion and hyperglycemic clamp to assess the effect of aldosterone on insulin secretion in normal subjects. We will then study subjects with the metabolic syndrome, a group at high risk for diabetes, to similarly assess the effects of treatment with MR and AT1 antagonism. These studies will have immediate clinical relevance to preventing the development of diabetes in individuals with the metabolic syndrome.